1. Field of the Invention
The present invention relates to recording method and apparatus, and more particularly to recording method and apparatus using an ink jet type recording head.
2. Related Background Art
Office Automation equipment such as personal computers and word processors have recently been popular widely and various recording apparatuses for printing out information inputted by such equipment and technologies to increase the speed of the recording apparatus and enhance the image quality have been developed rapidly. Typical examples of increasing the recording speed are the increase of the number of recording elements of the recording head and the control of block drive of the recording elements to efficiently drive the increased number of recording elements.
FIG. 11 illustrates the prior art block drive control of the recording elements of the recording head. In FIG. 1, n1, n2, . . . , n16 denote ink discharge nozzles of a recording head of an ink jet type and each nozzle is provided with a recording element therein. An image is recorded on a recording medium by ink droplets discharged from the respective nozzles. A dot formed by each ink droplet is called a record pixel. Usually, the number of nozzles provided in one recording head is 50 to several hundreds although FIG. 11 shows the recording head having 16 nozzles to simplify the drawing. In FIG. 11, lines drawn vertically at an equal interval represent a pitch (recording pitch) of the record pixels. For example, in a recording head having a recording density of 360 DPI (dots per inch), the pitch is approximately 71 .mu.m.
The nozzles of the recording head shown in FIG. 11 are arranged such that a direction of arrangement of the nozzles is oblique to a direction of feed of a recording sheet (medium) and the recording head is mounted on a printer at such an angle that an interval between the nozzle 1 (n1) and the nozzle 5 (n5) is just the recording pitch. The recording head scans on the recording medium to record the record pixels in accordance with the record data to form a record image. For example, when one-dot line is to be recorded in a column shown by an arrow designated by "record column" in FIG. 11, the recording element corresponding to the nozzle 1 (n1) is driven when the recording head comes to the position "1" in FIG. 11, and then the recording element corresponding to the nozzle 2 (n2) is driven when the recording head comes to the position "2", and similarly the recording element corresponding to the nozzle 16 (n16) is driven when the recording head comes to the position "16" so that the vertical one-dot line as shown in FIG. 11 is recorded.
In the recording head described above, taking the recording pitch into consideration, the nozzle 1 (n1), nozzle 5 (n5), nozzle 9 (n9) and nozzle 13 (n13) are grouped in one block because they are requested to conduct the record operation concurrently, and similarly the nozzle 2 (n2), nozzle 6 (n6), nozzle 10 (n10) and nozzle 14 (n14) are grouped in another block, the nozzle 3 (n3), nozzle 7 (n7), nozzle 11 (n11) and nozzle 15 (n15) are grouped in yet another block, and the nozzle 4 (n4), nozzle 8 (n8), nozzle 12 (n12) and nozzle 16 (n16) are grouped in a further block.
Namely, in the recording head of the above construction, since the concurrent drive of more than four nozzles does not occur, a cost is reduced by the decrease of a power supply capacity, compared to a case where all of the sixteen nozzles of the recording head are to be concurrently driven. Further, by arranging the recording elements across the plurality of record pitches a recording density of the record dots increases and a high grade record image is attached, compared to a case where the nozzles of the recording head are arranged vertically to a direction of movement of the recording head and all nozzles are concurrently driven to record one-dot line.
As means to attain the high grade image, a PWM drive control, for example, may be conducted in which a drive pulse to record one pixel is formed by a multiple of pulses as shown in FIG. 12 and a pulse width is modulated in accordance with a state of the recording head.
Further, technology to expand the resolution such as a smoothing process to expand in the recording apparatus the resolution of record data sent from an external apparatus such as a personal computer has been developed and put into practical use.
However, the high speed control and the high image quality control in the prior art are competing to each other.
For example, when the number of recording elements is doubled to double the recording speed, the number of blocks to be driven in one drive period increases when the number of concurrently driven nozzles is constant. As a result, when eight blocks are to be division-driven by a 6 KHz drive pulse, a drive time of approximately 20 .mu.s may be allocated to one block, but when 16 blocks are division-driven, a drive time of only 10 .mu.s is allocated to the drive pulse. Similarly, when the drive pulse frequency is to be doubled, the period of the drive pulse is halved and the same problem arises.
This is considered for a recording head having 128 nozzles by taking into consideration the number of nozzles of the recording head actually mounted on the printer (approximately 50 to 128). Assuming that the number of concurrently driven nozzles is 8 and the drive frequency is 160 .mu.s as they are for the recording head having 16 nozzles, the number of blocks is 16 and the heater drive time per block should be set to shorter than 10 .mu.s. Further, when the double resolution expansion control described above is to be conducted under this condition (for example, the recording resolution is to be expanded from 360 DPI to 720 DPI), the resolution in a main scan direction of the recording head is doubled (the recording pitch is halved or the number of columns per unit length is doubled) and the number of pixels to be recorded in a given time is doubled. As a result, the heater drive time per block should be set to shorter than 5 .mu.s. When the heater drive time is short, it is difficult to apply to the heater even a minimum thermal energy to cause bubbling of ink.
The pulse width modulation drive for correcting the ink discharge amount for high grade image or the recording head drive control for the multi-value image data recording may be attained without the resolution expansion control if the heater drive time is set to shorter than 10 .mu.s. But, the drive time is not sufficiently long and the setting of a longer drive time is desired. Thus, in order to attain the pulse width modulation control for the high grade image, the longer permissible drive pulse width is desired, which is contrary to the high speed requirement.
In the multi-value image which is considered to be a main technology in the future high grade image technology, it is necessary that the drive pulse is wide.
On the other hand, if the number of concurrently driven nozzles can be increased, the number of divided groups decreases and the heater drive time per group may be theoretically extended without changing a total record time. However, the number of concurrently driven nozzles has an upper limit when the requirements to reduce a capacity of a power supply or the stabilization of the voltage to be applied to the recording elements (heaters) are taken into consideration. Namely, it is desired to increase the number of concurrently driven nozzles without increasing the drive load.
For example, when a heat generating resistor which is a 119 .OMEGA. heater is driven by a 24-volt power supply, a current of approximately 200 mA flows per nozzle assuming that a resistive component for causing a voltage drop such as a wiring resistance is approximately 1 .OMEGA.. In this case, assuming that the heat generating resistors are connected in parallel to the drive power source and the wiring resistance for each heat generating resistor is connected in series to the drive power source and also that the number of concurrently driven recording elements is eight, a maximum current is 1600 mA. When the numbers of concurrently driven recording elements are one and eight, the voltage drops due to the voltage drop factors are 0.2 volt and 1.6 volts, respectively, and the power supply voltages applied to the heater are 23.8 volts and 22.4 volts, respectively. When the maximum number of concurrently driven recording elements is 16, the voltage drop due to the voltage drop factor increases to 3.2 volts from 0.2 volt for the one concurrently driven recording element and the power supply voltage applied to the heater decreases to 20.8 volts from 23.8 volts. If the drive condition (pulse width) is set to assure the sufficient discharge of the ink with the applied power supply voltage of 20.8 volts, the equivalent pulse is applied to the power supply of 23.8 volts and a load to certain heat generating resistor increases and the reliability and durability thereof are damaged.
By this reason, the number of concurrently driven recording elements is limited.
Further, it may be theoretically possible to drive the heater by a high voltage above 30 volts, for example, to reduce the pulse width. However, even with this method, an unlimitedly high voltage power supply cannot be used when a property of the transistor driver for driving the heater and a production cost of the recording head are taken into consideration. Further, when the high voltage drive is used, a high current flows through the transistor, the heater and the surrounding wiring resistance, the voltage drop due to the voltage drop factor increases and a variation of the power supply voltage applied to the heater increases.
In addition, it is apparent that the number of recording elements of the recording head and the drive period therefor will be reduced in order to enhance the quality of the recorded image and the resolution, so the coordination between the satisfaction of such requirement and the limit of the number of concurrently driven recording elements will be more and more difficult.
There are various techniques to realize the high grade image and it is not limited to the technology to control the pulse width to drive the recording head. But the high speed control technology and the high grade image technology are in an intimate relation from standpoints of both the stable use of the recording head and the most direct technology to control the record pixels.